Environmentally harmful species in the exhaust gas emitted from an internal combustion engine, such as hydrocarbons (HC), carbon monoxide (CO), particulate matters (PM), and nitric oxides (NOx) are regulated species that need to be removed from the exhaust gas. In lean combustion engines, due to the existence of large amount oxygen excess, passive means without extra dosing agents, such as that using a three-way catalyst, normally are not able to effectively remove the oxidative specie NOx as that in most of spark-ignition engines. To reduce NOx in lean combustion engines, a variety of active means with reducing agents (reductants) being dosed in exhaust gas are developed. In these technologies, normally the reductant is metered and injected into the exhaust gas, and the result mixture flows into a SCR (Selective Catalytic Reduction) catalyst, where the reductant selectively reacts with NOx generating non-poisonous species, such as nitrogen, oxygen, carbon dioxide, and water.
A variety of reductants, such as ammonia (NH3), HC, and hydrogen (H2) can be used in SCR systems. Among them, ammonia SCR is used most broadly due to high conversion efficiency and wide temperature window. Ammonia can be dosed directly. However, due to safety concerns and difficulties in handling pure ammonia, normally urea solution is used in ammonia SCR systems. Urea can be thermalyzed and hydrolyzed to ammonia in exhaust gas.
Reductants can be frozen under low temperature. For example, urea solution of 32.5% wt has a freezing temperature of −11° C. Therefore, in applications under low temperature, a temperature control is needed to maintain the working temperature above freezing point. Also, since frozen reductant may damage the mechanical components in a dosing apparatus, such as the pump and the injector, and the temperature control normally is turned off after dosing finishes, in order to protect the dosing apparatus, reductant residue in the mechanical components needs to be purged before they are exposed to low temperature for a long time. Additionally, in the temperature control, typically the reductant tank has a closed-loop heating control with a heater and a temperature sensor positioned in the reductant tank. However, for reductant passage lines, due to distributed heating, it is difficult to control temperature in closed loop and locally over-heating could damage the lines. To reduce the requirement for line heating performance and to increase the system reliability, reductant residue in the reductant passage lines also needs to be purged after a dosing process completes.
Normally, in a dosing apparatus, the pump is unidirectional. However, during purging, since most of the reductant residue in the dosing apparatus needs to be drawn back to the reductant tank to avoid urea deposit and crystallization in the exhaust pipe, to which the dosing apparatus is fluidly connected, a reverse reductant flow needs to be created, and a complex flow control is then required. For example, a method for reverting flow is swapping the inlet of the pump with the outlet. This method involves multiple flow changes, and since partial flow change would create a fluid loop with the inlet fluidly connected to the outlet, resulting in purging failures, in addition to being able to swap the pump inlet with outlet, the system design and purge control should also avoid the fluid loop from happening. Additionally, during purging, since the injector nozzle needs to be opened when the reductant residue in the injector is drawn back to the reductant tank, high temperature air in the exhaust pipe could come into the dosing apparatus, causing damage when it passes through the pump. To avoid this problem, before purging, the air temperature at the injector nozzle needs to be examined. However, after dosing completes, normally the engine is keyed off as well. Without exhaust flow, it is difficult to obtain an accurate temperature at the injector nozzle due to temperature distribution in the exhaust pipe. Furthermore, when air goes into the pump, the pumping efficiency drops significantly. It is also difficult to clean the passage lines, especially the passage line connected to the injector, which normally is the longest passage line in the dosing apparatus.
To effectively purge reductant residue, a primary object of the present invention is to provide a dosing apparatus with a purging means having a simple control that needs not create a reverse flow through the pump or use other fluid such as compressed air to push out the reductant residue into the exhaust pipe.
A further object of the present invention is to provide a purging means pulling back the reductant residue with a constant drawing pressure.
Another object of the present invention is to provide a purging control means that controls purging after a dosing process completes.